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A more recent version of this article appeared on November 1, 2002
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Submitted on May 2, 2002
Revised on July 24, 2002
Accepted on July 29, 2002
1 Center for Genetics and Development and Section of Molecular and Cellular Biology, University of California, Davis, CA, 95616
* Corresponding author. E-mail address: jmscholey{at}ucdavis.edu.
We proposed that spindle morphogenesis in Drosophila embryos involves progression through four transient isometric structures in which a constant spacing of the spindle poles is maintained by a balance of forces generated by multiple microtubule (MT) motors and that tipping this balance drives pole-pole separation (Sharp et al., 2000b). Here we used fluorescent speckle microscopy to evaluate the influence of MT dynamics on the isometric state that persists through metaphase and anaphase A and on pole-pole separation in anaphase B. During metaphase and anaphase A, fluorescent punctae flux towards the poles at 0.03 µm/s, too slow to drive chromatid-to-pole motion at 0.11 µm/s, and during anaphase B, fluorescent punctae move away from the spindle equator at the same rate as the poles, consistent with MT-MT sliding. Loss of Ncd, a candidate flux motor or brake, did not affect flux in the metaphase/anaphase A isometric state or MT sliding in anaphase B, but decreased the duration of the isometric state. Our results suggest that, throughout this isometric state, an outward force exerted on the spindle poles by MT sliding motors is balanced by flux, and that suppression of flux could tip the balance of forces at the onset of anaphase B, allowing MT sliding and polymerization to push the poles apart.
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